Apnoeic heart rate responses in humans. A review.

Research examining the heart rate response of humans to apnoea and apnoeic immersion in water has tended to produce equivocal results. Controversy exists in relation to the existence and onset, course and ultimate extent of apnoeic bradycardia, either at rest or during exercise. The reduction in heart rate that has been observed in response to apnoea and apnoeic immersion is considered by some to be a component of an oxygen conserving mechanism termed the mammalian dive reflex. Doubt has been expressed, however, as to whether humans do possess the complex cardiovascular response evident in diving mammals. A bradycardial response to apnoea at rest is reasonably well established. Reports conflict with regard to apnoeic responses to exercise in both terrestrial and aquatic environments, and for the temporal onset of apnoeic bradycardia in general, and the time course required for the maximal development of the response. It is suggested that the existing discrepancies with regard to heart rate responses to apnoea are due in part to the large variety of research protocols employed, with, in many cases, a lack of control of those factors said to modify apnoeic heart rates, and in part to a lack of uniformity in the interpretation of the obtained results. Conflicting evidence exists for almost all of the factors said to modify the apnoeic heart rate response in humans. Factors implicated in the phenomenon of apnoeic bradycardia include the influence of temperature and the physical condition of the individual, varying lung volumes, the depth of immersion in water, the body position during the apnoeic episode, and the psychological state of the individual. The influence of the sex and age of the subjects is equally contentious. Finally, the role of breath-holding capacity as it influences the extent of bradycardia remains to be examined further. Additional research requires the adoption of an integrative, holistic approach if a comprehensive understanding of apnoeic heart rate responses is to be achieved.